Fundamental investigations of thermal aging phenomena of model NO_x storage systems

摘要

Investigations of the aging behavior induced by high temperatures coupled with oxidizing atmosphere of model NO_x storage systems Ba/Al_2O_3 and Ba/CeO_2 are reported in this paper.The samples were prepared,calcined and exposed to temperatures between 500 and 1000degC in air for 12 h for thermal aging.Samples were characterized with XRD,HRSEM,DSC-TGA-MS and BET analyses.In XRD investigations of all model systems calcined at 500degC for 2 h,the NO_x storage component was present in form of BaCO_3.The release of CO_2 as a result of the decarbonization of the NO_x storage component at increased temperatures was verified by thermogravimetric investigations.In the case of Ba/Al_xO_3,already during calcination a partial reaction of the NO_x storage component with A1_2O_3 resulting in the formation of barium aluminate was observed.In the model system Ba/CeO_2 the decomposition of the barium carbonate started above 780degC and the formation of a barium cerium mixed oxide was observed.The presence of the barium containing NO_x storage component has a strong influence on the specific surface area of the model NO_x storage systems.The morphology and crystallite size of CeO_2 modified with the barium containing NO_x storage component exhibited distinct changes compared to the unmodified oxide.The NO_x storage efficiency determined by model gas tests of freshly prepared and engine aged model NO_x storage catalysts correlates well with the above described observations.